As the use of low quality refinery feedstocks has increased, a concomitant need for improved resid processing capacity has accompanied it as these feeds generally result in larger quantities of residual fractions in the refinery. At the same time, the long term needs to cut costs and to make cleaner products represent conflicting requirements. Feed accounts for about 70% of the refining costs and the use of less expensive feeds would cut costs. However less expensive feeds typically have higher sulfur, metals, and aromatics which make them more costly to process. Thus, in order to meet the objective of reducing costs, the heavier refinery fractions which contain the bulk of the sulfur, metals and aromatics must be processed more efficiently into the more valuable lower boiling fractions such as gasoline and distillate.
One of the many types of processes developed for the treatment of residual feeds is the hydroconversion of heavy residual feedstocks in a slurry process using a catalyst prepared in a hydrocarbon oil from a thermally decomposable metal compound catalyst precursor. The catalyst may be formed in situ in the hydroconversion zone or separately as described, for example, in U.S. Pat. Nos. 4,134,825; 4,226,742; 4,244,839; 4,740,489 and 5,039,392 which describe processes of this type using catalysts based on the metals of Groups IVB, VB, VIB, VIIB and VIII of the CAS Periodic Table (i.e., Groups 4-10 in the IUPAC Periodic Table (2004)), preferably from Groups VB, VIB and VIII (i.e., Groups 5, 6 and 8 through 10 in the IUPAC Periodic Table (2004)).
In the aforementioned process, it is possible to use hydrogen pressures which are far lower than the 1500-3000 psig (about 10,000-21,000 kPag) used in conventional hydroprocessing techniques. At these lower pressures, typically as low as 250 psig (about 1725 kPag), a substantial proportion, typically up to 65%, of 650° F.+ (345° C.+) resid molecules can be converted to lower boiling range products, e.g. 650° F.− (345° C.−) fractions, using a few hundred parts per million of a dispersed metal on carbon catalyst at 450° C. (about 840° F.). The small amount of catalyst is enough to maintain coke at a manageable level and the hydrogen pressure is low enough that aromatic rings are not saturated so there is low hydrogen consumption. A significant portion of the feed is converted to lower boiling range products (e.g., products which can be treated as in the 650° F.− (345° C.−) boiling range) which are high in saturated (aliphatic) molecules. The higher boiling range portion of the reaction products (e.g., the 650° F.+ (345° C.+) portion) can then be treated in separate processing in a way which utilizes the favorable characteristics of the hydroconversion products.